This invention relates to a gypsum wallboard and more particularly to a novel fiber reinforcement for the wallboard and to a novel process for constructing gypsum wallboard.
In the making of gypsum wallboard, it has frequently been the practice to employ reinforcing fibers in order to strengthen the wallboard. Such fibers are usually incorporated into the stucco slurry which is cast between the front and back paper cover sheets to make up the gypsum wallboard. The fibers tend to reinforce the core in much the same manner as the paper cover sheets which surround the core. These fibers span the local discontinuities in the stucco matrix and unite adjacent zones or agglomerates of matrix particles. The fibers, for instance, reduce the amount of core fragments that might otherwise detach from the board during nailing near an edge of the board where the core is exposed.
Of the many kinds of fibers which are available, only two have been widely used in gypsum board, paper fiber and glass fiber. The paper fiber is the more widely used type of fiber because it is cheaper, but the glass fiber has the advantage of being highly fire resistant and, therefore, is essential in the makeup of many gypsum products which have to meet certain fire specifications. While both types of fiber are used to provide "core integrity", in the event of fire, glass fibers can sustain core integrity for a longer, more critically useful period than paper fibers.
The diameter of individual glass filaments presently used as reinforcing fibers for gypsum board core is less than one-half thousandth of an inch. Because of their small diameter their tensile breaking strength is quite low, less than about 0.07 lb. Their size also makes them easily airborne. The handling of such filaments is facilitated by incorporating numbers of them into a strand called an " end", and combining a number of ends into a larger strand called "roving". For example, a glass fiber reinforcement roving may contain 60 ends, each of which consists of about 200 filaments.
Roving is usually chopped into nearly uniform lengths and conveyed to the gypsum board slurry mixer. These actions tend to separate the roving into a mixture of individual filaments and bundles of filaments. Further disaggregation, as well as dispersion and perhaps some fiber breakage, takes place in the slurry mixer. But the disaggregation is seldom perfect, so gypsum board core normally contains a mixture of individual filaments and bundles of filaments.
When fibers are incorporated into the gypsum slurry, they tend to slow the slurry flow thus slowing the manufacture of the board. In order to speed up the slurry flow to the proper level it is necessary to add excess water to the slurry. This is disadvantageous because: (1) the water added to the slurry must be eventually driven out in the drying process; and (2) the excess water decreases core density and this reduces fiber pull-out resistance.
A further limitation with respect to the fibers which might be incorporated into the gypsum slurry is that the fibers longer than about one-half inch cannot be introduced into present gypsum board slurry mixers because such longer fibers would become entangled and hang up on the pins and other parts of the machine. Moreover, if longer fibers were incorporated into the slurry for the core, they would tend to reinforce the core, but would result in a corresponding impairment in the scoring and breaking properties of the board. In the field when workmen wish to cut a gypsum board, they simply score it through the paper with a knife. The scoring does not usually penetrate deeply into the core and does not sever many of the fibers in the core. Thus, when the board core is snapped, the unsevered fibers that extend across the fracture tend to pull away core aggregates resulting in a very irregular edge which interferes with the fitting of adjacent pieces together and could be a source for complaint. While this situation could perhaps be somewhat minimized by increasing the core density of the board or changing the fiber geometry or using various agents to improve fiber-to-core "bond", none of these are presently practical approaches to improving the pull-out resistance of the fibers to effect a clean break across the scored portion of the board. Increasing the density of the core is impractical because the present market prefers lightweight gypsum board, and additives to improve the bond are not practical because they are presently very expensive. Finally, a change in a glass fiber geometry has been found to be presently uneconomical.
The present invention provides a unique approach to the solution of the foregoing problems. The invention eliminates the need for incorporating excess water in the stucco slurry. Moreover, the invention increases the strength of the board only where it is most needed and in the normally weakest (i.e., transverse) direction, and this strengthening is accomplished without greatly impairing the scoring and breaking properties of the board.